In my opinion, if you always taking hits to the knees, protect them as well as you can. You only get one set and, in my experience, they are costly to repair and are not usually the same after.
However, in saying that, I ride with soft shells.....really only you can decide if the added weight and possible discomfort of hard shells is worth it.
If I had the cash, I would definitely get one of those POC spine protectors though.
I know, not a very helpful response